I feel that my knowledge of SS and SD are fairly good. I have read and studied the CCTA books. Are the practice exams good indicators? That is if I score 90 on a practice exam should I feel the "level" at which the practice exam is at is closely matched to that of the real exam? Ultimately, How to know you are ready for the foundations exams?

You will never know that you are ready...but there's one thing that you should do...register for an exam, once you know the date, manage your time to take the exam.
Prouste wrote a book about that artist who were painting a women, he never finished his work.and you can see a hand on the pic, that was perfect...but the remaining was a mess....he wanted to have the work perfect...we are humans, by definition, we are not perfect...why do you wanna take the ITIL exam perfectly prepared...you just can't
Mock exams can be a good indicators but not always, but I will definitely recommend to take as much mock exams as you can...
I thought that I was not prepared...I scored high...